1. Field of the Invention
This invention relates to semiconductor devices and more particularly to such devices requiring programming (such as programmable read-only memory elements (PROM elements)) or incorporating redundant circuitry.
2. Prior Art
Demand for PROM elements, particularly in semiconductor large scale integrated (LSI) circuits has increased dramatically recently. The PROM element is used not only in LSI PROM elements but also in other LSI memories and LSI logic circuitry with redundant circuitry (fault-tolerant circuitry).
PROM elements may be programmed by any of the following techniques:
1. PN junction shorting technique, in which the PN junction is shorted by passing excessive current therethrough to accomplish the programming.
2. Current fuse programming technique as disclosed in U.S. Pat. No. 3,792,319 to Tsang. A fuse element is dissolved by heat generated from current passing therethrough to accomplish programming.
3. Laser fuse programming technique in which the fuse device is disconnected by employing an irradiating laser to write data.
The first and second techniques require a very large current to program the data. For example, a programming current value of about 100 mA is required for the first technique, and the programming current for the second technique is about 50 mA. A bipolar transistor having a large amplification is needed to generate such a large current. When these programming techniques are applied to devices using MOS transistors, the MOS transistors for generating such large currents require a very large channel width. Large areas in MOS.LSI.PROM are occupied by such MOS transistors. Therefore, it is difficult to fabricate LSI PROM with high component densities by the first and second techniques.
In the third technique, irradiation must occur very close to the active element. Very expensive equipment is required for automatically and accurately determining the irradiating position.
FIGS. 1 and 2 show a conventional fuse element of the second, current fuse technique, FIG. 1 being an equivalent circuit and FIG. 2 being a top plan view. In FIG. 1, the fuse element 100 is melted by passing current between terminals 102 and 104. In FIG. 2, fuse 100 is formed of a polysilicon layer. Aluminum wiring layers 106 an 108 are connected to fuse element 100 at terminals 110 and 112 through contact holes 114 and 116. These contact holes have a width greater than fuse element 100.
In this current fuse technique a few problems exist in addition to the difficulty of high packing density aforementioned. First, the melting mechanism is difficult to control. The necessary programming current varies from one device to the next. Second, elements near the fuse device may be damaged by the heat when the fuse is melted by the current. Third, the reliability of the fuse devices is low, because the fuse device is not covered with a passivation layer (protecting layer).